Hot Galactic Halo

A long-standing controversy regarding our own Milky Way Galaxy is the
origin of the diffuse X-ray emission which seems to extend throughout the
Galaxy. It's difficult to determine the exact 3-dimensional distribution
of this hot X-ray emitting gas since it's difficult to determine the
distance to the extended gas. Many astronomers have suggested that some of
this hot gas exists in a large halo around the Milky Way, and a new
observation with the Chandra X-ray
observatory of a spiral galaxy called NGC 4631 helps support this idea.
Using Chandra's exquisite resolution, astronomers have seen that some of
the X-ray emission from this galaxy originates in a diffuse cloud of hot
(2-7 million degree) gas which surrounds the galaxy. The image above show
the Chandra observation of the hot gas halo (seen in blue-white in this
false color image) superimposed on an image of the galaxy taken by the Hubble Space Telescope. The Hubble image
(in orange and white) is dominated by the emission from large groups of
stars in the plane of the galaxy, while the X-ray emission seen by Chandra
extends far from the galactic plane. It may be that the halo seen by
Chandra may be peculiar to NGC 4631, but more observations of other Milky
Way-like galaxies will help finally resolve the galactic X-ray halo
controversy.